The mobility and light weight of hand-held powered grinding wheel tools has popularized them in a wide variety of production line tasks such as deburring castings, smoothing weldments and shaping surfaces. Such portable grinders are generally denominated vertical grinders. A conventional vertical grinder includes a housing containing an air or electrically driven motor which directly drives a grinding wheel attached thereto. The housing generally includes at least one handle which assists the operator in moving and positioning the grinder and a guard which substantially encloses the grinding wheel and protects the operator from inadvertent contact with the grinding wheel.
The design parameters and operational requirements of grinders are relatively well established. One of the primary areas of concern is overspeed of the grinding wheel. All grinding wheels are rated for a maximum r.p.m. and it is incumbent upon tool designers to incorporate speed governors into their equipment to prevent rotational speeds in excess of these maximum rated grinding wheel speeds. Numerous devices and patents are directed to achieving speed governing and overspeed shutdown. Since the grinding wheels in such tools may rotate at 6000 and as much as 8000 r.p.m., the need to control rotational speeds is manifest.
Another problem related to high speed and the substantial rotational kinetic energy which the grinding wheel possesses at such speeds involves the motor and drive mechanism. Generally, grinders include a threaded shaft upon which the grinding wheel and various washers are placed and retained by a mating nut which is threaded onto the shaft and tightened against the wheel and washers. The thread direction on the drive shaft is such that relative rotation between the grinding wheel washers and nut produced by frictional drag of the grinding wheel against a surface causes the nut to tighten against the grinding wheel and washers rather than loosen. In a like fashion, if the grinding wheel and nut are loose, start-up of the drive shaft will tend to tighten rather than loosen the nut.
Unfortunately, the opposite operating conditions produce the opposite result. That is, a slowing or rapid stop of the motor and drive shaft relative to the grinding wheel tends to loosen and unthread the retaining nut and loosen the grinding wheel. Given the substantial kinetic energy present in the rapidly rotating mass of the grinding wheel, it is clear that not only does it contain sufficient energy to unthread the retaining nut but also to inflict substantial damage to whatever object it encounters if it dislodges from the drive shaft.